Cyclic oligomers formation

Ito, K., Hashizuka, Y., and Yamashita, Y., Equilibrium cyclic oligomer formation in the anionic polymerization of e-caprolactone, Macromolecules, 10, 821-824, 1977. 

Ha and Choun [51] confirmed these findings from the investigation of cyclic oligomer formation at 270 °C. They derived a rate equation for cyclic oligomer formation taking thermal degradation of the polymer into account. 

Peebles, L. H., Huffman, M. W. and Ablett, C. T., Isolation and identification of the linear and cyclic oligomers of polyethylene terephthalate) and the mechanism of cyclic oligomer formation, J. Polym. Set, PartA-1, 7, 479 (1969). 

Transfer and termination occur by the modes described previously for cyclic ether polymerizations. Chain transfer to polymer (both inter- and intramolecular) is facilitated in cyclic acetal polymerizations compared to cyclic ethers because acetal oxygens in the polymer chain are more basic than the corresponding ether oxygens [Penczek and Kubisa, 1989a,b]. Working at high monomer concentrations, especially bulk polymerizations, is used to depress cyclic oligomer formation. 

The anionic polymerization of D4, in toluene, at room temperature, with Li+ + [211] as counterion appears to follow a different course than in the case of D3. The rate of polymerization is much lower and cyclic oligomers formation is more important as can be seen from the results shown in figure 6 for [C] 2.76 10" mole.l". ... 

Table 3. Cyclic oligomer formation during the cationic polymerization of cyclic sulfides in CH2C12 as solvent...

Polymerization competes with cyclic oligomer formation... 

Further research is still required in this area to discover more effective and reproducible reaction conditions/catalyst systems. Such an approach could optimize formation of only the high molecular weight linear polystannananes with better structural control and suppression of cyclic oligomer formation. 

Little is known about the overall mechanism of cyclic oligomer formation, although the mechanism of the initial stages of the sequence seems fairly clear. The first chemical event is the reaction of formaldehyde (formed in the Petrolite procedures by depolymerization of paraformaldehyde) with phenol to form 2-hydroxy-methyl- and 2,6-6w(hydroxymethyl)phenols in a base-catalyzed process, as shown in Fig. 3. Such compounds were characterized many years ago50), obtained from the action of aqueous formaldehyde on phenol in basic solution at room temperature. Subsequent condensation between the hydroxymethylphenols and the starting phenol occurs to form linear dimers, trimers, tetramers, etc. via a pathway that might involve o-quinonemethide intermediates which react with phenolate ions in a Michael-like reaction, as portrayed in Fig. 4. The condensation of hydroxymethyl-... 

As shown in Sect. 3.2.1, the rate of cyclic oligomer formation (expressed by the probability of attaining the corresponding conformation) is proportional to n 3/2. Thus, for systems conforming to the Jacobson-Stockmayer theory, the distribution... 

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